>UPDATE. Levaminsole HCI can be used on crocea clams!!!<
I'd wait a few days or weeks (or maybe you have already, in that case I take back my comment) before being certain the clam was not harmed.
FWIW, in the past I've purchased clams from a few people that had all types of algae problems. I purchased one clam completely covered in Bryopsis and Valonia. I have found that you can remove a clam from water and force it to close completely. Then you can CAREFULLY spray the outside of the clam shell with tap water, then 70% ethanol (or 70 isopropanol, rubbing alcohol from the drug store), wait about 30 seconds, then rinse the shell again quickly with tap water, then quickly get the clam back into some salt water to rinse everything off. The treatment with alcohol effectively rapidly dehydrates and kills anything (algae, and any other critters) on the shell. You have to be careful of course not to get the tap water or alcohol into the opening in the clam (some species have larger openings than others). I've used this technique to clean the shell of several clams (T. deresa, and T. maxima) and also used it occasionally on snails that were coated in suspisiuos (wish I could spel) algae.
The reason for 70% rather than 100% is that for some reason it is more effective at killing than 100%. Hospitals, and other folks that need to kill things also use it at 70%.